Code-based multiplexing, such as used in Direct Sequence Code Division Multiple Access (DS-CDMA) wireless communication networks, allows for the transmission of potentially many different signals on the same frequency. In such contexts, receivers extract individual signals of interest from the “composite” received signal by correlating the composite signal with individual spreading codes used by the transmitter for those signals of interest. For example, an individual receiver recovers control, pilot, and data channel signals from the received composite signal based on despreading the composite signal using control, pilot, and data channel spreading codes.
Despreading different (code) channels at the same time requires the allocation of separate decoding resources to each channel. Simplistically, then, recovering three different code channels from a received composite signal requires three different “despreaders.” In actuality, most contemporary receivers assign more than one despreader to each code channel of interest, as part of multipath signal reception operations. For example, assuming three dominant propagation paths for the received composite signal, a given RAKE-type receiver might assign three despreading “fingers” to a data code channel, for data reception on each path delay, while simultaneously assigning another three despreading fingers for decoding the pilot channel at the same three path delays. The latter assignment permits the receiver to make propagation channel estimations for each of the dominant delay paths.
While the above simple example hints at the potential need for despreading resources, a more concrete example better illustrates the point. For example, wireless communication receivers, such as mobile terminals or stations of the type used in cellular communication networks, are becoming multimedia service terminals, providing a wide range of voice, data, and entertainment services to their users. Existing and developing wireless communication standards, such as the current and forthcoming releases of the 3GPP WCDMA standards, recognize and provide for these types of services. Particularly, the developing WCDMA standards provide support for video and medium-rate data (300 Kbs) (defined in Release 99), High Speed Packet Access (HSPA) service (defined in Releases 5/6) for high data rates (up to 4 Mbs on the uplink and up to 14 Mbs on the downlink). Other examples of rich, multimedia services transmission scenarios include those associated with the Multi-cast/Broadcast Multimedia Services (MBMS) defined in the Release 6 of the 3GPP standards.
Typically, each type of service requires at least one channelization code, meaning that one or more despreaders must be assigned to that code. Simultaneous services, along with associated control and signaling overhead, being received on multipath propagation channels thus translates into the need for a potentially large pool of despreading resources at the receiver, e.g., each code requires a dedicated despreader per each signal propagation path of interest. The requirement for potentially large numbers of individually assignable despreaders is particularly true in the context of Generalized Rake (GRAKE) receivers, which include Rake fingers operated as “probing” fingers for interference characterization, in addition to those Rake fingers dedicated to despreading the code channels of interest.
In the HSPA context, the channelization codes a given receiver is obligated to despread might include those for a Common Pilot Channel (CPICH), Broadcast Channel (BCH), Dedicated Physical Channels (DPCHs) from up to six cells, up to fifteen High Speed Packet Data Shared Channels (HS-PDSCHs), and up to four High Speed Shared Control Channels (HS-SCCH). Furthermore, a number of downlink control channelization codes for the uplink may also be needed, such as E-AGCH and E-HICH/E-RGCH. Multiplying the total number of code channels involved by the number of radio paths of interest requires a potentially large number of despreaders, e.g., likely something well in excess of 100 despreader resources per receiver branch.
The despreader resource requirements multiplicatively increase as additional receiver antennas and receiver front-ends (branches) are added, such as for diversity reception improvements. For example, if, for one receiver branch, 100 despreaders are required to despread all of the channel codes of interest at all of the propagation path delays of interest, then twice that number—i.e., 200 despreaders—generally would be required for two receiver branches.
One approach to reducing the number of individual despreaders needed relies on doing more with them. For example, increasing the speed at which the despreading process operates by a factor of two means that two code channels can be recovered from a buffered received signal by one despreader in the same amount of time needed by two despreaders operating in parallel on the signal but at half the speed. However, higher processing speed generally means higher power consumption, greater complexity, and greater expense.
Of course, the actual number of despreaders needed at any given time depends on the particular communication services being used and the prevailing radio conditions. However, because despreader resources generally represent “fixed assets” in the receiver, designing and building a receiver with fewer than the maximum number required for a worst-case scenario, compromises receiver performance, at least during worst-case conditions.